It seems as if the water in the glass is defying gravity, but is it really? No. Gravity – the force pulling down on everything – is still at work even when the glass and water are above your head. The water’s inertia wants to keep the water traveling in a straight path, but gravity is acting on the water, causing it to fall in a downward path that will eventually hit the earth.
However, while the water is falling, the glass is falling with it, catching the water. What keeps the glass and water moving in a nice circular path that doesn’t get wet or messy is the string.
The string acts as the centripetal force that pulls the glass and water into the center. But watch out. In order for the glass to keep falling with the water, the glass must travel fast enough to keep up with the water. If you spin the glass too slowly, the water will fall out and you will get wet.
It’s not magic that keeps people in roller coasters that travel in looping, spiraling paths – it’s physics. Try this experiment to see how centripetal force and inertia keep people inside cars even when traveling upside down.